Biomanipulation of the ecology of earth ponds to stimulate the production of brown trout (Salmo trutta L.)

A study of various biomanipulation strategies to maximise plankton and macroinvertebrate production in earthen ponds for the benefit of cultured brown trout, Salmo trutta L., was carried out in the Howietoun fishery in Central Scotland between March 1988 and January 1990. During the first year, replicate ponds were treated with low and high phosphorus only (LP, HP), high phosphorus and nitrogen (HPN), low and high chicken (LC, HC), high chicken and cow manure (HCC), with two untreated controls (CTRL). The effect of treatments on physico-chemical parameters of pond water and soil, suitable for trout culture was also evaluated; along with primary production. All the inorganic treatments produced significantly more Bacillariophyceae, Chlorophyceae and Cyanophyceae than the CTRL, but HPN produced vastly more algae than the others. All inorganic treatments stimulated zooplankton, but the advantage of HPN was far less significant than with algae. The organic trials responded to all treatments, but more positively to HC & HCC in the zooplankton. The role of decaying plankton and organic organic manure as sources of detritus and carbon, providing nutrients for optimum biological production, is discussed. The application of HC and combination of HCC gave the highest benthos production, with the dominant groups encountered in the order: Oligochaeta > Chironomidae > Asellidae > Sialidae > Hirudinea > Mollusca, all eaten by trout. Total abundance, biomass and dry weight production production estimates during the 6 month’s culture period were in the range, 3.51 —134 x 10* ind. m**, 26-113 g m* and 14.7-70 g dry wt m** respectively. Nutritional composition varied among the natural groups, but was well within nutrient requirements for cultured trout. Aquatic macrophytes in some ponds favoured production of Asellidae, Mollusca and lately, Gammaridae, presumably due to greater three-dimensional surface area which influences development of plankton. Overall fish growth response conformed to a seasonal cycle, and significantly varied between the CTRL and ponds that received the same fertilizer treatments (HPN, HC and HCC). Highest annual production estimates of 619.7 kg ha** yr ** and 1439.6 kg ha** yr** was obtained in the HCC, pond 1 (fed natural food only) and CTRL pond 8 (fed artificial pelleted diet, with supplemental natural food) respectively. The beneficial effects on abundance and biomass .of plankton and benthos exerted by the fertilization treatments, compatible with adequate water quality for salmonids, is examined in relation to feeding conditions and growth of juvenile trout stocked at various densities during the following summer. Cost-benefit analysis of the controlled and manured culture conditions shows that for each pound spent, a benefit or economic impact of £3 : 78 and £6 : 40 respectively is derived. Practical management implications and economic considerations of pond ecosystem manipulation under both temperate and tropical conditions are discussed in the light of the present findings